Purified microvascular endothelial (MVE) cell preparations from guinea pig fat pad, heart, and brain were found to contain the histamine degrading enzymes, histamine methyltransferase (HMT) and diamine oxidase (DAO). The enzyme content of these cells accounted for most of the HMT activity in the whole fat pad, and a significant part of the enzyme activity in heart and brain. High HMT activity was also found in heart myocytes and gastric parietal cell, which like the capillary endothelial cell are important sites of histamine action. In addition to the histamine degrading enzymes the MVE cells contained a small pool of histamine that was resistant to compound 48/80 and appeared to turn over at a rapid rate. The data indicate that MVE cells contain a nonmast cell store of histamine as well as histamine in activating enzymes.